Organic isocyanates are generally produced on the commercial scale by reacting the corresponding amines with phosgene. Because of the toxicity of phosgene, attempts have been made for a long time to find a commercially practical method of synthesis for organic isocyanates, in which it is not necessary to use phosgene. One such method of synthesis involves the reaction of organic nitro compounds with carbon monoxide and organic hydroxyl compounds to form the corresponding urethanes, and the subsequent splitting of the urethanes formed into compounds containing isocyanate groups and compounds containing hydroxyl groups. In this type of process it is also possible to modify the urethane obtained as intermediate product, before splitting it up. Thus, it is possible, for example, to initially react the phenyl urethane obtainable from nitrobenzene, carbon monoxide and ethanol with formaldehyde to form the bisurethane of 4,4'-diisocyanato diphenylmethane and then to convert the intermediate product thus obtained into 4,4'-diisocyanato diphenylmethane, with the elimination of the ethanol.
The splitting of urethanes into compounds containing the corresponding isocyanate groups and corresponding hydroxyl groups is described, for example, in German Offenlegungsschrift No. 2,421,503 and in the publications mentioned in that reference.
Essentially two types of catalysts for the production of the urethanes are described in the patent literature. The reaction of organic nitro compounds with carbon monoxide and alcohols to form urethanes in the presence of selenium or selenium compounds is described for example in German Offenlegungsschriften Nos. 2,343,826; 2,614,101 and 2,623,694. In these reactions, good urethane yields are obtained both with mono and with dinitro compounds. The selenium compounds, and in particular the organo selenium compounds formed as intermediate products during the reaction and hydrogen selenide, are extremely toxic and have to be removed quantitatively during the working up treatment, for example by means of a chemical reaction. This generally results in a complicated chemical working-up stage and means that the process is of doubtful economic viability. Noble metals, in particular palladium, in the presence of Lewis acids are described as catalysts in German Offenlegungsschriften Nos. 1,568,044 and 2,603,574. Anhydrous iron (III)-chloride is mentioned as a particularly effective Lewis acid. Although good urethane yields are obtained with these catalysts, relative to the amount of nitro compound used, the yields are unsatisfactory with respect to the amount of hydroxyl compound used. Thus, large proportions of diethyl ether are obtained because of the acidic properties of the Lewis acid, when ethanol is used as hydroxy component. Corrosion of the refined steel autoclaves used as reaction vessels is also observed when using these noble metal/Lewis acid catalysts. The corrosion can be limited to a considerable extent by adding organic bases such as, for example, pyridine, as described in German Offenlegungsschrift No. 2,603,574, but an unacceptably large amount of ether is still formed in the presence of these catalyst systems. A further disadvantage of these catalyst systems is their poor capacity for regeneration since the Lewis acids used are not sufficiently stable in the presence of the hydroxy compounds used.
It has now surprisingly been found that the reaction of organic nitro compounds with carbon monoxide and organic compounds containing at least one hydroxy group to form urethanes can be carried out in the liquid phase at elevated temperature and elevated pressure in the presence of palladium and/or palladium compounds and a cocatalyst consisting of mixtures of iron oxides and/or iron hydroxides and activating chloridecontaining additives.